1. Field of the Invention
The present invention relates to image forming apparatus in an electrophotographic system such as a printer and a color photocopier.
2. Related Background Art
In recent years, it is desired for color image forming apparatus in an electrophotographic system such as a color printer and a color photocopier to provide output images with a high image quality. In particular, density gradation and stability thereof affects image quality levels much.
However, any changes in respective parts of apparatus due to environmental changes and long use will fluctuate image density of color image forming apparatus to be obtained. In particular in case of color image forming apparatus in an electrophotographic system, even a tiny environmental change might give rise to a density change to bring about color disbalance so it is necessary to have means for keeping a constant temperature. Therefore, a toner image for density detection (hereinafter referred to as patch) is configured to be made into an intermediate transfer member and a photosensitive member so as to detect the density of the pre-fixed toner patch with a pre-fixed toner density detecting sensor (hereinafter referred to as density sensor), feedback the detection results thereof to process conditions such as exposure quantities and developing bias to control density and thereby to obtain a stable image.
However, the density control with the density sensor is to form the patch onto an intermediate transfer member and a drum and the like to detect, and does not control changes in image color balance caused by performance changes in successive transfer onto the transfer member as well as in changes in the fixing nature. Density control with the density sensor cannot cope with these changes.
Therefore, a color image forming apparatus (hereafter referred to as color sensor) equipped on transfer material with a sensor to detect density or color of the patch on the transfer material has been proposed (for example, refer to Japanese Patent Application Laid-Open No. 2003-287934).
This color sensor is configured of three or more kinds of light sources with different emission spectrums such as red (R), green (G), blue (B) and the like as light emitting elements or by a light source emitting white (W) as light emitting element to form three or more kinds of filters with different spectral transmission factors of such as red (R), green (G), blue (B) and the like on a light-sensitive element. Thereby three or more different kinds of outputs such as R, G and B are obtained.
This color sensor is used to implement image density control (image gradation control) to realize more accurate density control.
However, the image forming apparatus described in the background of the invention is accompanied by the following defects.
In case of detecting toner image density (or chromaticity) on transfer material with the color sensor, the distance between the color sensor and the transfer material needs to be kept constant. The reason is that any change in distance between a color sensor and transfer material will generally vary detection output values to no small extent. That is, unstability in the distance between the color sensor and the transfer material will worsen detecting accuracy to disable good density control.
On the other hand, in order to stabilize the distance between the color sensor and the transfer material, the transfer material needs to be supported from the both front and rear surfaces, and when the supporting members (rollers, plate members and the like) for such a purpose contact the post-fixing toner image, gloss of that portion will vary compared with the other portions to constitute an image defect. In general, the contacting members rapidly cool and press the toner image in the portion where the supporting members are brought in contact and therefore gloss of that portion will get more intensive than in the other regions to constitute gloss unevenness with vertical streaks. In addition, in order to stabilize the transfer member posture, the contact pressure of the contact members should be made higher to some extent, that is, gloss unevenness is apt to occur than in case of sheet conveying member (a sheet discharging roller and the like) for transfer material discharge with comparatively light pressure setting. In addition, since, at the time of double-sided printing, a toner image exists in the rear surface of the transfer material as well, the transfer material supporting member brings about a similar defect.
That is, image forming apparatus equipped with a color sensor for detecting toner image density on transfer material used to give rise to a bad image called gloss unevenness.